\begin{fmffile}{propagatorefotone}
\begin{fmfgraph*}(100,100)
\fmfleft{i}
\fmfright{o}
\fmf{boson,tension=1.2}{i,v1}
\fmf{boson,tension=1.2}{v2,o}
\fmf{plain,left}{v1,v2,v1}
\end{fmfgraph*}
\end{fmffile}
